Field of the Invention
The invention relates to a method for frame error detection in a data transmission, and to a device for performing the method.
In order to make maximum use of the limited capacity of an available transmission channel, before data transmission in the transmitting direction, especially in video, audio and speech transmission, a source coding is done in order to eliminate redundance from the data and thus compress the data to be transmitted. Often the data to be transmitted are first divided up into so-called frames. These frames can be time slots of a certain duration or can also be spectral ranges or image excerpts or segments. Then in the source coding, parameters are ascertained that describe or determine the data to be transmitted as accurately as possible. This is known as parametric source coding. One example of it in the case of speech coding is GSM full-rate coding. In speech source coding (speech coding), the ascertained parameters are also called speech coefficients.
In the transmission of these compressed data, if the channel conditions are poor, errors can occur despite protective provisions such as channel coding; then the received speech, for instance, is greatly interfered with and may even becomes unintelligible. In the GSM standard, all the received speech frames are therefore first classified as good or bad with the aid of a frame error detection criterion. The bad frames are then replaced with a frame, which typically results from extrapolating one or more previously received "good" frames. The interference in the speech can thus be reduced, and the speech intelligibility is preserved.
In GSM full-rate transmission, for instance, the speech is subdivided into frames of 20 ms each, and each frame is compressed into 260 bits by the full-rate or enhanced full-rate coder. These bits are ranked by their importance as class 1a, 1b, and class 2. The 50 bits in class 1a are secured, among other things by way of a parity check, with 3-bits. If this CRC (cyclic redundancy check) shows an error, then the frame is declared bad. The 3-bit parity check in the full-rate speech coder has a certainty of approximately 7/8 for error detection; that is, good frames are found to be bad and vice versa. Bad frames that have been detected as good are often heard as a pronounced "crackling" in the speaker of the mobile phone.